I’ve been married for 23 years during which my husband and I have raised four sons, and have had plenty of rocking and rolling in our relationship. From my own experiences, and from the dozens of sagas unloaded into my tape recorder, I am constantly reminded of the eggshell-thin line that separates loving from loathing. I know that staying married can mean plates flying across kitchens, tears soaking pillows and emailing old boyfriends at 3 a.m.

I thought nothing could shock me about what really goes on behind closed doors between two people working hard to make it “til death do us part” — without killing someone first. After all, I have heard every brand of twisted love story — swinging, adultery, spouses coming out as gay after 30 years together, threesomes, fist fights in restaurants, even the tale of a husband discovered to be having sex with a sheep, documented in a photograph discovered by his wife in his nightstand drawer.

But in piecing together this latest book I have been surprised at some of the revelations. I’m not as ruffled by the tawdry tales of farm animals or one I heard from a 55-year-old wife about screwing a perfectly sculpted landscaper while her doctor husband was lecturing on vein surgery in another country. My biggest shock is how many outwardly cheerful women who have been married forever think about divorce if not weekly, at least once a month.

How’s this for a statistic? Of the 200 plus women interviewed and woven into The Secret Lives of Wives, I can count on one hand those who have never considered splitting up. It was no surprise that Beth often considered leaving her husband. He routinely told her she was fat and ugly, and when they fought in the car he would pull over and shove her out the door. Who could blame Shauna for her many consults with a divorce lawyer? She’s the wife of the traveling doctor, a man who hasn’t initiated sex since their honeymoon 30 years ago. Her secret is that she has it both ways: an intact family and a ten-year affair with a hard-bodied lover, who does her landscaping for free.

The biggest shocker is the number of wives in stable unions who frequently contemplate fleeing their marriages. These are not abused wives; they are women with nice husbands who give them orgasms and jewelry and stability. Yet many of these settled midlife women admitted they were slightly jealous of Tipper Gore who gets to have a fresh start after 40 years of matrimony with the same guy. While many speculated about whether one of the Gores fell in love with someone else, my instincts without talking to either of them is that perhaps they are a lot like other couples portrayed in the book. Maybe they were simply sick of being around each other. And maybe one or both of them finally couldn’t take it any more.

Who stays married and who doesn’t is a question not always about commitment or deep abiding love — it’s about endurance.

I have found in my collection of wives who remain in long running marriages that the majority of them share these common traits: They have the guts and determination to stick it out, no matter what. And their laments about their marriages aren’t because of anything serious. It’s the subtle nuances of living with one person in one house for a very long time that grates at the soul, that causes a simmering malaise. It’s the grind of the ordinary that drives people into thinking, “Is this all there is? I want more. I want adventure. I want change.”

Who wouldn’t want changes with the current statistics on lifespan? Women in their 80s and 90s are the fastest growing segment of the aging population which means that many of us wives could easily hit our 50th wedding anniversaries and beyond. That’s a hell of a long time to sustain one love affair, particularly when empty nest hits and it’s only you and the husband with no cushion of kids as a buffer.

There are three strategies that have worked the best with the women I interviewed. The happiest wives have a sense of purpose and passion in work and causes outside of the home. Wives who counted on a spouse for fulfillment and sustenance were often angry and lonely. And the happiest wives don’t spend a whole lot of time with their husbands. My chapter called Separate Summers is filled with women who take their own vacations, take their own summers, take charge of their own lives. Couples who allow each other to grow separately are the ones with the best chance of growing together and staying together.

Finally, the wives with the highest marital satisfaction have a tight circle of wild women friends with whom to drink, travel and vent about their husbands.

Yes, my work on this book has been quite surprising and enlightening. I now know that acceptance of mediocrity in a marriage relationship is more prevalent than you would imagine. I know that sometimes the only reason women stay with a spouse is because they have divorced friends who may have more sex than they do with new husbands but they also have cranky step-kids who hate them. Other women stay in lackluster marriages because they don’t want to give up their swanky lifestyles, and divorce is expensive, really expensive. We know from our friends who are pushed to the edge and do call it quits that the grass isn’t always greener, there are parched patches on both sides of the fence.

But most women told me they stay married simply because they like their marriages more than they dislike them, even if much of the time it’s 51 percent “like” to 49 percent “dislike.”

Iris Krasnow is a bestselling author and an assistant professor in the School of Communication at American University. Connect with her on: www.iriskrasnow.com